Update component build documentation.

docs/component_build.md

 # The Chrome Component Build
 
 ## Introduction
 
 Release builds are “static” builds which compile to one executable and
 zero-to-two shared libraries (depending on the platform). This is efficient at
 runtime, but can take a long time to link because so much code goes into a
-single binary. When you set the GN build variable
+single binary.
+
+In a component build, many smaller shared libraries will be generated. This
+speeds up link times, and means that many changes only require that the local
+shared library be linked rather than the full executable, but at the expense of
+program load-time performance.
+
+The component build is currently the default for debug non-iOS builds (it
+doesn’t work for iOS). You can force it on for release builds using the
+[GN build arg](https://www.chromium.org/developers/gn-build-configuration):
 
 ```python
 is_component_build = true
 ```
 
-the build will generate many smaller shared libraries. This speeds up link
-times, and means that many changes only require that the local shared library
-be linked rather than the full executable, but at the expense of program
-load-time performance.
-
 ### How to make a component
 
 Defining a component just means using the GN “component” template instead
 of a shared library, static library, or source set. The template will
 generate a shared library when `is_component_build` is enabled, and a static
 library otherwise.
 
 ```python
 component("browser") {
   output_name = "chrome_browser"
@@ skipping 213 matching lines @@
 cases above where `_IMPLEMENTATION` is inappropriately defined or inappropriately
 undefined. Use GN visibility to make sure callers don’t screw up.
 
 ### Putting exported symbols in static libraries
 
 As discussed above, exported symbols should not be in static libraries because
 the object file might not be brought into the link. Even if it is brought in
 today, it might not be brought in due to completely unrelated changes in the
 future. The result will be undefined symbol errors from other components. Use
 source sets if your component is made up of more than one target.